A prior art activating device in which the frequency of a voltage applied to a piezoelectric element is preset to a frequency which is higher than an intended drive frequency and is higher than the resonating frequency and is lower than the harmonics of the resonating frequency, and is gradually lowered is disclosed in, for example, Japanese patent Kokai publication JP-A-3-22873. This is a driving device of an ultrasonic motor for preventing the occurrence of uncomfortable noise on activation of the ultrasonic motor. This prior art device will be described with reference to FIG. 16.
A plurality of electrostrictive elements of polarized piezoelectric material are disposed on the surface of an ultrasonic motor stator 34. These plurality of electrostrictive elements are separated into two groups, section A and section B, which are disposed in such a manner that they are out of phase by 90 degrees to each other. The electrostrictive elements of sections A and B are provided with electrodes 47a and 47b, respectively. An electrode 47c is disposed on electrostrictive elements which are insulated from the electrodes 47a and 47b so as to output a detecting signal depending upon the oscillating condition of the electrostrictive elements.
When a voltage from a power source is applied to this circuit, a microcomputer 40 outputs data concerning an upper limit of the driving frequency to a VCO 38 via a D/A converter 39. This causes a voltage of the upper limit frequency to be applied to the electrode 47a via a frequency divider 37, amplifier 36 and a coil 35. Simultaneously with this, a voltage having a frequency which is out of phase by 90 degrees from that applied to the electrode 47a is applied to the electrode 47b. The electrode 47b is driven by a frequency given by "a shift" register 44 having inputs connected to both sides of the frequency divider 37, and an output connected to an amplifier 45 and further to a coil 46.
However, since the upper limit frequency is far from the optimal drive frequency, the ultrasonic motor is not activated and no signal is obtained from the electrode 47c. Accordingly, a band pass filter (B.P.F.) 43 outputs a 0 level signal, which is fed to the microcomputer 40 via a rectifier 42 and an A/D converter 41. The signal is compared with a predetermined reference value Ds in the microcomputer 40 for detecting that desired driving is not performed. In response to this result, the microcomputer 40 outputs data for presetting the drive frequency to a frequency which is lower by 0.1 KHz.
A series of above-mentioned operations is repeated until a signal which is input to the microcomputer from the electrode 47c via the band pass filter 43, rectifier 42 and the A/D converter 41 becomes the predetermined reference value Ds, until then drive frequency is continued to lower.
The value of Ds is preset in such a manner that the signal input to the microcomputer 40 is higher than the reference value Ds when the drive frequency is in the vicinity of the resonating frequency after lowering of the drive frequency is continued. When the signal input to the microcomputer 40 is actually higher than the reference value Ds, the microcomputer 40 outputs data for increasing the drive frequency by 0.1 KHz. When the signal input to the microcomputer 40 is lower than the reference value Ds again, the microcomputer 40 outputs data for lowering the frequency correspondingly. Such a series of operations enables control of drive frequency so that the signal input to the microcomputer 40 converges to the vicinity of the reference value Ds.
Since the frequency of the voltage applied to the piezoelectric material is not identical with the resonating frequency on activation of the ultrasonic motor by the above-mentioned operation, uncomfortable noise does not occur.